1. Field of the Invention
This invention relates to an inductance L load actuating apparatus that actuates an inductance L load of a stepping motor or the like.
2. Related Art
A conventionally known apparatus, as this kind of apparatus, is for example disclosed in the Japanese Utility Model No. HEI 7-33598, published in 1995. The stepping motor apparatus, disclosed in this prior art, reference will be explained in greater detail hereinafter.
FIG. 22 shows an actuation circuit for a bipolar winding 2-phase stepping motor. A direct-current power source unit 1, generating a voltage Vcc, is connected to each of first coil 2a to fourth coil 2d. These first coil 2a to fourth coil 2d are connected to first to fourth switching elements (MOS transistors) 3a to 3d, respectively.
A first current-detection resistance 4a is connected between a common connecting point of switching elements 3a, 3c and the ground. A diode 5a is connected in parallel to the first current-detection resistor 4a for forming a current path for discharging energy stored in the coil. In the same manner, a second current-detection resistance 4b is connected between a common connecting point of switching elements 3b, 3d and the ground. A diode 5b is connected in parallel to the second current-detection resistor 4b.
Responding to exciting current signals supplied through output lines 7a and 7b of an exciting signal generation circuit 7, first and second control circuits 6a and 6b, respectively control the switching elements 3a to 3d in accordance with a predetermined exciting method (e.g. 2-phase exciting method). More specifically, control circuit 6a compares the current detection signal obtained from a detection line 8 connected to the first current-detection resistance 4a with a reference voltage V1 produced from a reference voltage generation circuit 10, and executes the intermittent control (i.e. chopping control) for the associated switching elements 3a and 3c. Similarly, control circuit 6b compares the current detection signal obtained from a detection line 9 connected to the second current-detection resistance 4b with a reference voltage V2 produced from the reference voltage generation circuit 10, and executes the intermittent control (i.e. chopping control) for the associated switching elements 3b and 3d. Through the above-described chopping control, a stepping motor is actuated by a constant current.
FIG. 23 shows waveforms of various portions of the above-described actuation circuit obtained when the stepping motor is actuated by the two-phase exciting method. Waveforms (A) and (B) represent exciting signals, while waveforms (C) through (F) represent gate signals of switching elements 3a through 3d, respectively. When the exciting signal shown by the waveform (A) is in HIGH level, switching element 3a is subjected to the chopping control. When the exciting signal shown by the waveform (B) is in HIGH level, switching element 3b is subjected to the chopping control. Furthermore, when the exciting signal shown by the waveform (A) is in LOW level, switching element 3c is subjected to the chopping control. When the exciting signal shown by the waveform (B) is in LOW level, switching element 3d is subjected to the chopping control.
Control circuits 6a and 6b have the same arrangement. FIG. 24 shows the detailed arrangement of control circuit 6a. Operation of control circuit 6a, during an exciting period for generating a signal to activate coil 2c, will be explained with reference to the timing chart of FIG. 25.
When switching element 3c is in an ON condition, the voltage applied between both ends of current-detection resistance 4a, i.e. current-detection voltage Vr, becomes a positive voltage in proportion to the current flowing through this resistance 4a during its ON period. On the other hand, when switching element 3c in an OFF condition, current-detection voltage Vr is equalized to the forward voltage drop of diode 5a. Accordingly, as shown by the waveform (A) of FIG. 25, current-detection voltage Vr varies in response to ON and OFF of switching element 3c.
A comparator 61 compares the current-detection voltage Vr with the reference voltage V1. When the current-detection voltage Vr reaches to the reference voltage V1, the output of comparator 61 is turned to LOW level. Thus, a capacitor 62 is discharged. When the current-detection voltage Vr is smaller than the reference voltage V1, the output of comparator 61 is in HIGH level. Thus, capacitor 62 is charged. The terminal voltage V.sub.62 of capacitor 62, hence, varies as indicated by the waveform (B) of FIG. 25. This voltage V.sub.62 is compared with a predetermined reference voltage V.sub.0 at a comparator 63, and is converted into a pulse signal V.sub.63 as shown by the waveform of FIG. 25. This pulse signal V.sub.63 is level-inverted through a NOT circuit 64.
In an exciting period of coil 2c, the signal produced from exciting signal generation circuit 7 is in LOW level and the output of NOT circuit 66 is in HIGH level. Hence, the pulse signal is supplied to switching element 3c through AND circuit 65. A voltage V.sub.GS, having the waveform (D) of FIG. 25, is applied between the gate and the source of switching element 3c.
Accordingly, as shown by waveforms (A) through (D), when the switching element 3c is turned on at the time t1 and the current-detection voltage Vr has reached the reference voltage V1, the output of AND circuit 65 is turned to LOW level and the switching element 3c is turned off. Thereafter, capacitor 62 is charged. When the terminal voltage V.sub.62 reaches the reference voltage V.sub.0 at the time t2, the output of AND circuit 65 is turned to HIGH level and switching element 3c is turned on. By repeating this operation, switching element 3c is subjected to the chopping control.
During the ON period (t2 to t3) of switching element 3c, current Ic shown by the waveform (E) of FIG. 25 flows through a circuit consisting of power source unit 1, coil 2c, switching element 3c, current-detection circuit 4a and the ground. During the OFF period (t1 to t2) of switching element 3c, energy stored by the excitement of coil 2c is discharged through coil 2a which is electromagnetically coupled with this coil 2c. In other words, current Ia flows through a circuit consisting of coil 2a, power source unit 1, bypass diode 5a and a built-in diode of switching element 3a. When the flowing direction of current Ia is defined by the arrow shown in FIG. 24, current Ia has the waveform (F) shown in FIG. 25. The composite current I of current Ic and current Ia is shown by the waveform (G) of FIG. 25.
The above-described operation is performed in the exciting period of any other coils 2a, 2b and 2d, in the same manner as in the exciting period of coil 2c.
As described above, current Ic flows in response to turning-on operation of switching element 3c. In this case, a voltage Vc (refer to the waveform (H) of FIG. 25) at a connecting point C between switching element 3c and coil 2c is a low-level voltage equivalent to the sum of an ON-voltage of switching element 3c and current-detection voltage Vr. A voltage Va (refer to the waveform (I) of FIG. 25) at a connecting point A between switching element 3a and coil 2a becomes a high-level voltage induced in proportion to the gradient of current Ic due to mutual induction between activated coil 2c and coil 2a.
When switching element 3c is turned to the OFF condition, the voltage Vc is increased to the high-voltage level by the energy stored based on the excitement of coil 2c. At the same time, in the coil 2a paired with coil 2c, a negative voltage is caused with a voltage drop equivalent to voltage Va by the energy stored in coil 2c. As a result, switching element 3a, having being set in the cutoff condition, is turned on with inverse bias, thereby supplying exciting current to coil 2a.
Accordingly, when switching element 3c is turned off, energy stored in coil 2c is discharged only through the coil 2a which is paired with coil 2a.
With the above-described operation, according to the above-described conventional system, voltage Vc at the connecting point C is increased up to a very high-voltage level in response to the transition of switching element 3c from ON to OFF. For this high voltage, it is necessary to increase the withstand voltage of switching elements 3a through 3d. Furthermore, emissive noise may be caused by the high-voltage surge.